Various types of aerators and agitators have been designed for use in commercial sewage treatment plants to enhance the breakdown of waste material in water through the aerobic process. Aerobic bacteria in the water require a constant supply of oxygen, and such equipment is designed to continually replenish the water with dissolved oxygen and, preferably, to increase the oxygen concentration to a level which substantially reduces the retention time of the waste water in the pond.
Three principal techniques have been utilized for accomplishing aeration of waste water for municipal or industrial applications. (1) Air diffuser units introduce oxygen near the bottom of a tank through a porous diffuser, such as sintered pipe. Air diffuser units achieve low gas concentration efficiency, and maintenance is high due to plugging problems. (2) Surface aeration units spray waste water in droplet form into the air, thereby increasing the gas concentration of the droplets before falling back onto the pond. Although commonly used, surface aeration units, as shown in U.S. Pat. Nos. 2,271,446 and 4,465,645 do not achieve good tank mixing, are generally expensive to operate and maintain since the units lift the water upward and water falls back onto the unit, and do not achieve the desired high gas concentrations in the waste water. (3) Submerged aeration systems employ a power rotating propeller beneath the surface of the water to push the water downward and achieve mixing while air is released above or below the rotating blades to mix and thereby becoming entrained in the water. These units generally also do not achieve high efficiency, and only small quantities of air per unit time are added to the water. Moreover, much of the air added to the water is retained in bubbles sufficiently large to rise to the surface and escape from the water rather than being entrained for consumption by the aerobic bacteria.
Submerged aeration systems generally fall into one of several classes: U.S. Pat. Nos. 4,240,990, 4,280,911 and 4,306,221 disclose aeration apparatus wherein the propeller is not radially enclosed within a housing, and air propelled by a motor fan is discharged downwardly from the surface to the propeller to mix with the water. U.S. Pat. Nos. 3,788,233, 3,975,469 and 4,448,685 disclose techniques wherein the propeller is similarly not radially enclosed, although air is drawn into the water by the Venturi effect caused by the rotating propeller, and is discharged immediately upstream or axially at the position of the propeller. The apparatii in U.S. Pat. Nos. 4,259,267, 4,437,765 and 4,290,885 also relies upon the Venturi effect to draw air into the water, although the propeller is enclosed within a generally tubular housing.
In an effort to increase the efficiency of submerged aeration systems, some devices employ a hollow propeller blade design, e.g., U.S. Pat. Nos. 2,651,413, 3,846,516, 4,193,949 and 4,265,739. These devices discharge gas through the ends of the blades, and some devices rely upon cavitation to draw air from the surface to the ends of the blades. These devices are, however, more expensive to manufacture, and maintenance is high due to the deleterious affect of cavitation.
The disadvantages of the prior art are overcome by the present invention, and improved methods in apparatus are disclosed below for safety and efficiently dissolving gas, such as oxygen, in a body of liquid to enhance aerobic treatment.